37 Degradation of Fungicides

Meenakshi Nandal

Objectives

Degradation mechanism of fungicides
Agents Involved in Degradation of fungicides
Microbial, Chemical and Photo degradation Processes

Introduction

A steady increase had been seen during the last two decades in the usage of fungicides. In general terms, chemicals used to control fungus are called “soil fungicides”.

“Soil fungicides” can be defined as the chemicals possessing low or negligible phytotoxicity and they protect the underground plants parts from attack by pathogenic soil fungi by preventing the multiplication of these fungi.

While, “soil fumigants” are defined as the chemicals that generally consist of high biotoxicity that could help to reduce or eradicate the harmful soil microflora and microfauna. The fungicides fall in the following four categories

(a) Seed protectants: used for disinfection or disinfestation of seeds and gets dissolved easily and get consequently incorporated into the soil. They are found suitable to protect the seeds from attack by the soil-borne pathogens (organic mercurials, chloranil etc.)

(b) Foliage spray fungicides: They have been found to possess a very high efficiency and stability, as tested under soil conditions (TMTD, zineb, captan).

(c) Soil fumigants: They have properties as an effective biocidal fungicide as found when testedwith regard to fungi in soil (chloropicrin, methylbromide, DDT).

(d) Modified fungicides: Finally, some fungicides especially develop with modifications through time and become available for the control of soil fungi (p-dimethylamino-benzene-diazo sodium sulfonate (dexon) and 2-propene-1,1-dioldiacetate).

As we know that soil pathogens can generally survive over any space as well as in any time series, so the fungicide has to operate in all the conditions according to space and time. The compounds like Captan, Nabani, Zineb with a low vapour pressure in suspension or in solution do not produce fungitoxic vapours. While, others are immediately disintegrated to vaporous substances in soil e. g. Mylone and Vapam. Considering the time dimension the survival of pathogenic forms can be constricted by immediate killing effects or by a delayed pathogen inhibition in soil. The effect of a soil fumigant is the result of concentration of the fungicide multiplied by the time period. In the soil the action of both factors depends on biotic and abiotic influences. The process of fumigation follows three stages:

(1) Radial diffusion where the radiation from the injection zones show high losses through the soil surface (time period- 1 day)

(2) A transition stage (for 2-4 days) where all the transformations and conversions takes place.

(3) The last third stage, in which the concentration start reaching the relative uniformity stage in the soil profile and it lasts till the fumigant is dissipated completely from the soil The soil fungicide acts in a monoculture like condition on the aboveground parts of a plant and the total soil micro flora can influence the action of fungicide in a positive or negative way.

Fungistatic properties

The underlying principle of the mode of action of fungicides is the inhibition of pathogenic fungal strains by affecting their reproduction and growth. This is not similar in case of soil fungicides compared to foliage fungicides, as the dispersal of spores is less in the soil than in air. Persistence: Another important criterion for soil fungicides is persistence. After checking the available literature, it seems that considerable confusion prevails on this point and principally a soil fungicide can persist with the ability to control a disease or a fungus. Furthermore, it can remain stable in a biological form or as a chemical. If only the biological indicators are used to analyze persistence, it cannot define how effectively the decomposition product is responsible for its activity.

Test fungi: A fungicide test may be impelled by the inoculum potential. Inoculum potential is defined as the age, nutritional stage, and phase of the activity among all the essential factors. Test plants: The angiosperms and fungi have common metabolic processes in general; step by step investigation of the principal differences in the metabolic pathways has to be done. Therefore, it is difficult to locate substances with a high antifungal activity with lower phytotoxicity.

Formulation: The fungicide is formulated as solution, emulsion dust or suspension. There are lesser known and available water-soluble fungicides and they are rapidly decomposed in soil. The absorption pattern of these fungicides is strongly influenced by their physico-chemical properties. Therefore, it is recommended to apply fungicides mixed with soil than in water mix form. Also, to fully utilize the high initial action of these fungicides and also to overcome the short lifespan, a mixture of soluble and insoluble fungicides can be used to attain both rapid and delayed control. Suspensions of fungicides are retained in the top layers of the soil, where they may cause serious phytotoxic injuries by the accumulation of high concentrations in this layer.

Environmental factors: Fungicides are highly influenced by the environmental factors, crop and fungal growth but it is difficult to distinguish the intensity of the several effects exerted by these factors. Generally, when the temperature is low, the host growth is slow and favors the damping off of these numerous fungi drastically. As a result, the fungicide function is slow and comparatively low at lower temperatures. The infection caused by these fungi is temperature-dependent and it will be better to use fungicide-combinations to be least affected by temperature. Moisture content: The rate of pathogen metabolic activity is influenced by moisture content, humidity and temperature of the soil as they influence the degree of sensitivity of pathogen and the diffusion of the chemical.

38.1 Degradation Reactions

1) Oxidation Reactions – The oxidation pathways are extremely important transformation reactions as they increase the pesticide solubility, alter parent activity, affect the bioactivity, mobility and enhance the environmental significance of chemicals There are two mechanisms of oxidation: physical and chemical often enhanced by light, heat or oxidized metals in soil or water involving molecular oxygen or more reactive species like various acids, peroxides, or singlet oxygen. Enzymatic activity accomplishes the biological oxidation by the mixed function monoamine oxidases, oxidases (utilizing cytochrome- P450) and the flavin-dependent monooxygenases. The hydroxyl radical, superoxide anion and hydrogen peroxide are the most reactive species. The mechanisms of some oxidation procedures are shown below:

This reaction occurs in pesticides generally in polycyclic aromatic hydrocarbons (PAH) and on aromatic rings in pesticides. It is easily executed on unsubstituted or activated single or multiple-ring molecules as with bulky or multiple groups. Substitutions like halogens can inhibit the reaction.

d) O-dealkylation: α hydroxylation is the process of Dealkylation of groups larger than methyl followed by the ether bond cleavage while O-dealkylation are generally species-specific.

In this the stereochemistry of the parent phosphorothion is retained by oxons (P=0) when the sulfur atom is oxidatively attacked with the metabolism of the R and S optical isomers of fonofos.

h) Sulfoxidation: The process of sulfides to sulfoxide by oxidation is quite rapid and is a result of the in vivo reaction by FAD monooxygenase enzymes. The cytochrome P450 monooxygenase forms sulfoxide and both these forms could also be generated by peroxides as well.

This process shows least resistivity towards degradation and results in oxidative metabolites having biological significance. One such group is aromatic alkyl thioethers, e.g., measurol and methiochlor.

2) Hydrolysis reactions– This process is a primary method for degradation of many pesticide The esters undergo hydrolytic cleaving resulting in two fragments with very less or no pesticidal activity. However, the products could possess bioactivity of many other types like microbial toxicity or microbial growth induction. Chemically the esters can hydrolysis by even mild alkaline solutions while strong acidic solutions (pH 3-4) are required to hydrolyse acids degradation.

a) Hydrolysis of phosphate esters

Biologically, two mechanisms are responsible for parathion hydrolysis one by hydrolases and other by oxidation, attributed to action of mixed function oxidases.

b) Hydrolysis of carboxyl esters

In case of malathion, rapid rendering is done by carboxyl esterase enzymes of vertebrates and process is non-toxic throughout hydrolysis. This enzyme is also possessed by malathion resistant insects. Example: Carbamate esters- methylocarb sulfoxide

3) Reductions Reactions: The products generated are with lower polarity and their biological activity may be concordantly altered. Reducing environment favour fungicide degradation generally characterized by presence of anaerobic microorganisms with low pH’s and oxygen concentrations. For example, stagnant or eutrophic ponds and lakes, bogs, flooded rice fields etc.

Reductive dehalogenation

(a) biotransformation of DDT in mammalian intestine

(a) biotransformation of Parathion flooded soil
4) Synthetic reactions: These conjugation reactions include addition of a moiety, relatively a polar one to a xenobiotic, primarily being a hydroxyl, carboxylic acid, amino or sulfhydryl group. The moiety is generally phosphate, glucuronic acid, glucose or amino acids. There is reduction in polar character of the moiety by alkylation (methylation) and reduction of the hydrophilic behaviour by acylation. The conjugation reactions can also affect the persistence, bioaccumulation and mobility of the pesticides.
5) Rearrangements: These reactions involve structural rearrangement in the molecule.

Isomerization – these are transformation reactions transforming one isomer to another under varying physical or chemical agents. Also, reversal in activity of diastereomers occurs in some cases. In case of synthetic pyrethroid insecticide, Fenvalerate two chiral carbons, one in the alcohol moiety and one in the acid portion of the molecule is present. The chiral carbon with alcoholic moiety undergoes racemization, till it reaches a stereochemical equilibrium while no change is observed in the acid-moiety chiral carbon.

38.2 Microbial degradation of thiocarbamate

Microorganisms in soil play a major role in hydrolysis of thiocarbamate at the ester linkage forming a mercaptan and a secondary amine. Prior to entering the metabolic pool, the mercaptan could be transformed into an alcohol by transthiolation, further oxidation could be done to form an acid. Such a mechanism, i.e., hydrolysis followed by transthiolation support the results observed in persistence and degradation studies on diallate. The diallate ester linkage hydrolysis proceeded by allylic group transthiolation results in formation of 2,3-dichloroallyl alcohol. (Figure 38.2)

thiocarbamate 38.3 Photodegradation of thiocarbamate(fungicide)

Photodegradation of thiocarbamates is not a very usual process. When exposed to sunlight minimum effects were noticed on compounds in the solid state, because of poor light. The light absorption produces two radicals after the breakage of the carbonyl C-S bond. They then form formamide and mercaptan after combining with protons from the solvent. Further degradation of formamide by ultraviolet radiation (UV) forms dialkylamine with carbon monoxide elimination. A disulfide is reformed after collision of two mercaptan radicals with continuous exposure to sunlight as the sulfur-sulfur bond is quite susceptible to photolysis. (figure 38.2)

38.4 Microbial degradation of Carbendazim

Carbendazim is a widely used broad-spectrum benzimidazole fungicide and is generally preferred for controlling a wide range of fungal pathogens on cereals and fruits. It is also used in treatment of soil and foliar application on the appearance of disease in crops. The fungicide carbendazim was degraded by amicrobial consortium obtained from several soil samples in paddy fields with continuous culture enrichment. Biodegradation using immobilized bacterial consortium was investigated in various parameters, as temperature, pH, and nutrient concentration. The degradation ability of the consortium was increased by immobilization on loofa (Luffa cylindrica) sponge, in comparison with that of free-living consortium. This immobilized consortium on loofa sponge is a promising material for bioremediation of polluted water with these pesticides in paddy fields. (Figure 38.3)

38.4 Microbial degradation of Carbendazim

38.5 Degradation of Fungicide Captan: The fungicide captan was degraded by the bacterial strain B. circulans through initial hydrolysis to produce 1,2,3,6-tetrahydrophthalimide, resulting in the loss of its fungicidal properties. There were some other products of this hydrolysis; H2S, CO2, HCl, 1,2,3,6-tetrahydrophthalimide formation in water and by soil microbes. This cis-1,2,3,6-tetrahydrophthalimide was again metabolized by the B. circulans releasing intermediates

cis- 1,2,3,6-tetrahydrophthalamidic acid, protocatechuic acid and o-phthalic acid. Then via ortho-cleavage pathway protocatechuic acid in presence of higher activities of protocatechuate 3,4-dioxygenase was further oxidized in the cell-free extract by B. circulans that was grown on captan. As a result, fungicide captan was completely degraded by B. circulans as shown in Figure 38.4. It could be concluded that these bacterial strains could be potentially utilized for bioremediation of soils that have been contaminated with toxic fungicides, causing adverse effects on soil microflora and microorganisms enrolled for nitrogen cycling in agricultural soils.

38.6 Microbial degradation of the fungicide Benomyl: The degradation of Benomyl occurs by hydrolysis releasing Carbendazim (Methyl-2-benzimidazole carbamate), an active registered ingredient (US.EPA 2002).

It is highly persistent in soil where the residues might even be detected after four-five years of application. It has a half-life of 3- 6 months but when it is applied to bare soil it increases to a year or so. Being a fungicide with broad-spectrum and long environmental persistence is a human carcinogen and highly toxic to living organisms. In an experiment was determined how the Benomyl affect soil microorganisms with elucidation of their role in the fungicide degradation after NPK addition. To per 600g of soil,375 mg N, 187.5 mg P and 187.5 mg K of NPK was added. After this benomyl was applied in three different concentrations; 0.032, 3.2 and 8.0 mg/g soil. The microbial counts in the soil and benomyl residue were determined at different intervals during 360 days study. It was concluded that benomyl has a significant stimulating effect on the multiplication and growth of different microorganisms. The microbial strains with highest microbial counts percent were reported to be in Pseudomonas sp. followed by fungi. This increased microbial count reflects positive relation to benomyl degradation rates. The degradation mechanism of Benomyl has been shown in figure 38.5.

38.7 Factors affecting fungicide performance

The persistence of fungicides is affected by six processes in environment: volatilization, abiotic degradation including photodegradation or pH activity, plant uptake, biotic degradation by microbial metabolism , movement in water based on solubility and sorption/ desorption to surface of plant and soil. When a fungicide is applied, the product meets with different fates. Temperature mainly governs these factors (plant uptake, biotic degradation, abiotic degradation) out of the six processes affecting fungicide persistence in the environment. It influences depletion as well as physical removal of a fungicide while mowing. Fungicides are essential, valuable tools, but they do have their own limitations but when used in conjunction with appropriate agronomic practices they could work. The best way to learn about the effect and degradation pathway of fungicides is to learn the chemistry and the diseases caused by them affecting soil.

Summary

We studied the degradation mechanism of fungicides.

We study the various properties of fungicides and their mode of action

We investigated the mechanisms of microorganisms and their enzymes during degradation process.

Various examples of fungicide degradation.

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References

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Megadi, V, Preeti N. T, Sikandar I. M and Harichandra Z. N (2010) Bacterial Degradation of Fungicide Captan, J. Agric. Food Chem. 58, 12863–12868, DOI:10.1021/jf1030339
Patnaik, P. A Comprehensive Guide to the Hazardous Properties of Chemical Substances, John Wiley & Sons, 2003, ISBN 978-0-471-71458-3, New Jersey, USA.
Rosman, Y., Makarovsky, I., Bentur, Y., Shrot, S., Dushnistky, T and Krivoy, A. Carbamate Poisoning: Treatment Recommendations in the Setting of a Mass Casualties Event, American the Journal Emergency of Medicine, (9),1117-1124, 2009, ISSN 0735-6757.

Web link:

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